{ INFORMATICS.MCCME.RU
  Problem statement: http://informatics.mccme.ru/moodle/mod/statements/view3.php?chapterid=321
  Verdict: Accepted
}
uses SysUtils, Math;
 
var
	i,n:longint;
	ans:extended;
	k,t:int64;
 
begin
reset(input,'input.txt');
rewrite(output,'output.txt');
read(n);
k:=3;
ans:=1;
t:=-1;
for i:=1 to n do begin
	ans:=ans+t/k;
	inc(k,2);
	t:=-t;
end;	
ans:=ans*4;
write(ans:0:9);
end.